Ferguson cannot afford for United's concentration to waver against the Blues after they hauled themselves back level with Barclays Premier League leaders Manchester City in midweek.
Stamford Bridge is the next stop in an arduous sequence of fixtures which will determine the shape United will be in for the title run-in.
Ferguson realised when he met the Londoners in September that Andre Villas-Boas had introduced a new method of playing.
"That was a crazy game," said Ferguson as he reflected on the 3-1 win.
"It could have been 20-18. I don't think it will be like that on Sunday. It will be much tighter."
Five months on, Chelsea still lack consistency. Yet they retain a formidable home record against the Red Devils, against whom they have not lost in Premier League action since 2002.
"It's not going to be an easy game," said Ferguson.
"In the last seven years there have been many terrific battles with Chelsea.
"The manager has brought in two or three different players and I noticed when we played them earlier in the season there was a change in the style from previous Chelsea teams.
"When a new manager comes in on the back of someone as successful as Carlo Ancelotti, changes maybe take time.
"Everyone has their different philosophy and different ways of doing things.
"That is what we are seeing with Chelsea at the moment."